A proposal is made to study the fundamental neuronal mechanisms by which visual cortical neurons with identified receptive field characteristics respond to intracortical and surface stimulating electrodes as a function of both cortical and retinotopic stimulation distance. The long term objective of these studies is to provide quantitative estimates of both the spatial and temporal resolving characteristics which might be acheivable in a cortical visual prosthesis for the blind. In the proposed series of experiments, single neurons in the visual cortex of the anesthetized cat and monkey will be isolated, and their receptive fields will be plotted on a tangent screen. A second microelectrode located at a nearby penetration site in the visual cortex and at a variable intracortical depth will be used first as a multi-unit recording electrode to assess average receptive field locaton, and then as a stimulating electrode for delivering capacity-coupled constant current pulses either singly or in brief trains. The threshold current required for neuronal activation as well as other parameters such as latency of activation and efficiency of cell driving will be determined as a function of both stimulation depth and inter-electrode separation distance expressed in both retinotopic and spatial dimensions. An analogous study using two-point stimulation from a pair of intracortical or surface electrodes will also be carried out and the differential effects of simultaneous versus sequential patterns of stimulation on single neurons will be measured.